The diaphragm-type electrolytic cell for the production of chlorine and caustic is one of the most common types of electrolytic cell currently in use for commercial production of these valuable chemicals. Generally, a diaphragm cell incorporates a plurality of parallel, vertically oriented anodes placed between parallel vertically oriented foraminous cathode tubes. The anodes utilized are generally of the dimensionally-stable type comprising a cylindrical anode riser made of titanium or titanium clad copper to which a pair of parallel foraminous titanium plates or screens are welded. Various designs of this dimensionally stable anode either place the screens in a fixed position relative to each other or allow movement of the screens toward and away from each other in parallel planes. The screens are generally made of a valve metal or alloy of a valve metal such as titanium and have applied thereto an electrocatalytic coating which lowers the discharge overpotential for chlorine produced in the electrolysis process and increases the lifetime of the anode in the highly corrosive environment of the anode compartment of an electrolytic cell. The electrocatalytic coatings are generally precious metals or oxides thereof or mixtures of nonprecious and precious metals and/or their oxides.
The cathode tubes generally comprise a foraminous structure which may be either a perforated plate, expanded metal mesh, or wire screening, with iron or steel being the most common material used for such cathode tubes.
A separator, which is generally applied to the exterior of the cathode tubes, is interposed between the anodes and cathodes. The separator may be a hydraulically permeable diaphragm comprising asbestos fibers or a mixture of asbestos and polymeric fiber materials, or the separator may comprise a hydraulically impermeable ion exchange membrane.
In a hypochlorite cell or a chlorate cell, no separator is used in a cell which is otherwise of substantially the same configuration as the above-described diaphragm cell.
The cathode tubes are generally connected at their side edges to a conductive cathode can, the can forming a four-sided box which is open at both the top and the bottom thereof. In the assembly of the electrolytic cell, the cathode can is lowered over the anode cell base which has the anodes vertically position thereon, and a sealing gasket is provided between the edges of the cathode can and the cell base to prevent electrical shorting of the components. A brine head cover atop the cathode can completes the cell assembly.
A typical cell base and anode assembly is generally described in U.S. Pat. No. 3,591,483 and U.S. Pat. No. 3,707,454, both to Loftfield et al. The cell bases as described by Loftfield et al, comprise an electroconductive base portion which may be of copper, aluminum, or iron having a series of holes drilled therein for accepting extended base portions of the anode risers and attaching such risers to the cell base. A nonconductive sheet of rubber or passivated titanium is placed over the conductive cell base which electrically insulates the cell base and seals the base from the brine electrolyte so as to prevent corrosion of the base by the brine contained within the cell. In a manner similar to the cell base, the base cover has a series of holes extending therethrough which correspond to the holes of the cell base for allowing the passage of the anode posts therethrough to the cell base. A flange may be provided which is located on the anode riser and above a threaded portion of the anode riser which attaches the riser to the cell base rests on the cell base cover. When a rubber cell base cover is used, compression of the cell base cover with the attachment of the anode riser to the cell base creates a compression seal between the flange and the cell base cover which prevents leakage of brine around posts.
As used in this specification, the term "passivated" as applied to valve metals in general and titanium in particular, will be understood to mean an electrolytically inactive surface coating of oxide which is formed on the surface of the valve metal. Most commonly, a passivated surface is formed almost immediately in situ by the action of electrolyte on the exposed surface of valve metals. Other methods of passivating a valve metal surface may also be used.
In the case of a passivated titanium cell base cover, it was necessary, as taught by the above-mentioned Loftfield et al, patent, that a compressible rubber gasket was required between the flange portion of the anode riser and a titanium cell base cover so that proper sealing could be effected.
It has been found over the years of utilizing the cell base and anode structure described above, that rubber components such as a rubber cell base cover or rubber gaskets surrounding the anode flange when titanium cell base covers were used, would deteriorate and cause a leakage of brine through to the cell base and result in substantial corrosion of both the anode risers and the cell base. During cell operations, rubber gasketing material is attacked by all of the very corrosive chemicals within the electrolyte such as chlorine, sodium hypochlorite, sodium chlorate, oxygen and sodium chloride with this corrosive attack being accelerated by high temperatures within the cell which can exceed 200.degree. F. Such attack necessitates frequent replacement of rubber parts within the anode base assembly thus requiring the complete disassembly of the electrolytic cell including the removal of anodes from the base. Should any of such rubber parts fail during operation, there is a consequent massive attack by the electrolyte on the metal components of the cell base.
Lifetimes of electrocatalytically coated anodes within a diaphragm-type electrolytic cell may be as much as ten years in the current state of the art. However, the need for frequent renewal of rubber parts within the anode base assembly, requires a much more frequent disassembly of the cell than would be necessary for the replacement of coated anodes. A sealing arrangement which would eliminate the use of rubber materials and the consequent required regular replacement thereof, would be desirable in that the anode base assembly would not have to be disassembled for any reason for a period of up to or more than ten years.
Many early and current cell designs avoid any leakage problem with the conductive base by providing a valve metal base cover which is completely integral, that is having no holes therein and welding connector plates, generally having an L-form to the side of the base cover facing the interior of the cell. Assemblies of this type are described in U.S. Pat. Nos. 3,956,097 and 4,118,306 and British patent specifications Nos. 1,125,493 and 1,127,484. The difficulty with these types of anode base assemblies is that there is considerable electrical resistance between the conductive cell base through the titanium base cover to the anodes themselves. The passage of current through the titanium base cover offers substantial resistance to the flow of anodic current. Also, it is necessary that good contact be maintained between the titanium base cover and the conductive cell base. This must be accomplished by the use of extremely clean, flat surfaces on the facing portions of the cell base and the base cover. The difficulties with this procedure are apparent.
One means for overcoming the difficulty of passing current from a cell base through an integral cell base cover to the anodes has been overcome by the use of perforated cell base covers with extended portions of the anodes passing through the perforations so that direct contact may be made with the conductive cell base. This reduces the electrical resistance of the system, but it creates the problem of keeping the highly corrosive electrolyte away from the cell base and the conductive extended portions of the anode posts. Electrolyte corrosion quickly destroys the cell base and creates a leakage problem requiring extensive repair or replacement of cell components.
While rubber gasketing offers a temporary solution to this problem, as noted above, it is still necessary to disassemble the cells on a regular basis to replace rubber gasketing materials which degrade during the operation of the cell. A more permanent and noncorrosive seal would be helpful.
Buoy et al, in U.S. Pat. No. 3,928,167 and related Pat. No. 3,891,531 describe a welded seal around anode posts passing through a perforated cell base cover made of titanium. The method involved welding a cup-shaped disk of titanium to a portion of the anode post so as to create an outwardly extending flange having an upwardly standing ring portion located at the outer edge of the flange. The titanium cell base cover has an enlarged perforation therein having a similar upstanding ring portion associated with the edge of the perforation. The diameter of the cup-shaped flange is approximately that of the perforation so that when the anode post is inserted into the cell base, the ring portions of the flange and the perforation are in alignment adjacent to each other and final sealing is effected by welding the two ring portions together circumferentially around the top of the perforation. While this method eliminates the use of rubber gasketing materials to create a seal between the electrolyte and the cell base around the perforation in the titanium cell base cover, at least two problems of assembly are created by this method. First of all, alignment of the perforations with the connecting holes in the cell base is absolutely essential so that the rings of the flange and cell base will come into proper alignment when the anode post is installed. There is little or no room for adjustment. The second difficulty is that when anode posts having screens attached thereto are utilized, the efficacy of welding along the top of the cell base cover and the flange portion of the cup-shaped disk becomes very difficult due to the space limitations imposed by the anode screens and adjacent anodes.
Crippen et al, U.S. Pat. No. 4,121,994, offers another solution to the sealing of anode posts to a titanium cell base cover. This patent discloses the use of a titanium washer which is welded to the anode post so as to create a flange in a manner similar to that described in Buoy et al. When the anode post is inserted into the cell base for electrical connection, the flange then rests on top of the perforated titanium cell base cover. The edges of the titanium washer-flange are then welded to the top of the perforated titanium cell base cover to create an impermeable seal around the base of the anode and the perforated cell base cover. Since there is no necessary alignment of upstanding ring portions of the apparatus as in Buoy et al, the problems of alignment are avoided. However, since the washer-flange is welded to the top of the titanium cell base cover, there is still the problem similar to that of Buoy et al of spacial interference between the anode screens and adjoining anodes which precludes the use of automatic welding equipment which could greatly facilitate the installation of anodes and guarantee uniformity of welding and sealing.
Additional problems associated with welding of anode posts to a metal cell base cover include the development of stresses with the uneven heating of the materials during welding and during cell operation when there can be an expansion or contraction of cell components. Such expansions and contractions can cause cracking in both the welds and various cell components, such cracking leading to leaks of electrolyte which can cause corrosion of cell components.
It is therefore a principal object of this invention to eliminate the use of degradable rubber components in the anode and base assembly for diaphragm-type electrolytic cells.
It is a further object of this invention to provide an anode and base assembly which can be assembled utilizing automatic welding equipment.
It is a further object of this invention to provide an electrolytic cell having a perforated cell base cover and direct attachment of anode posts to a conductive cell base wherein critical alignment of the anode posts with perforations in the perforated cell base cover are not critical and sealing around the base of the anode posts is effected by welding the anodes to the metallic cell base cover.
It is yet another object of this invention to provide a welded titanium cell base cover in which anode posts are welded to the cover with connecting portions of the anode posts passing therethrough which is uneffected by heating distortions caused by both welding and by high temperature cell operation.
These and other objects of the invention will become apparent to those skilled in the art upon the reading and understanding of the drawings and specification presented hereinafter.